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Non Vented Vaulted Roof Assembly

 
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Old 06-19-2015, 09:27 PM   #1
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Non Vented Vaulted Roof Assembly


OK, so I have some friends that have decided to build their own tiny house. I've been able to help guide them in several of their building questions. However, we both currently live in Florida, and they will eventually be hauling their place up to North Carolina at some point after it's finished. Currently they are considering their insulation options. It will be a vaulted ceiling and the roof is constructed of 2x4 rafters as the span of the tiny house is right around 8'. There will be T&G ceilings applied directly to the underside of the rafters for the ceiling finish. It has a peel & stick dry in / roofing underlayment with a metal roof. So those are all of the known's.

Now in considering their insulation options they'll be using a spray foam, but they can't decide on whether or not to use open cell or closed cell. There have been some reports, that open cell foam is allowing humidity inside of the house during colder months (which we don't really experience here, hence my lack of knowledge on this question they posed to me) to pass through it and saturate the roof decking causing it to rot out. These reports suggest that closed cell foam is the only way to proceed to avoid this.

Now I will reiterate that its a tiny house, and its all of about 180sqft. My suggestion was to use open cell, as they are trying to maintain a budget, and to spring for a dehumidifier to use in the winter months.

Can any of you guys living in areas that experience condensation issues in colder months weigh in on this for me?

Thanks in advance.
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Old 06-19-2015, 10:09 PM   #2
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Re: Non Vented Vaulted Roof Assembly


Well if they still can, use ridged foam board on top of the deck but I believe you said it is roofed already?

Even so it may not be a big deal to take off the metal roof and put the foam (much less expensive than spray) on the deck then re-roof.

Then fill the bays with non-faced FG or more foam board.

Andy.

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Old 06-19-2015, 10:15 PM   #3
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Re: Non Vented Vaulted Roof Assembly


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Well if they still can, use ridged foam board on top of the deck but I believe you said it is roofed already?

Even so it may not be a big deal to take off the metal roof and put the foam (much less expensive than spray) on the deck then re-roof.

Then fill the bays with non-faced FG or more foam board.

Andy.

It's only dried in currently.

So, you're suggesting rigid foam with the metal installed over it? And then batt insulation in the bays, opposed to spray foam? Would they need to vent the roof then? Also, there aren't any eaves due to travel / size regulations.
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Old 06-19-2015, 10:20 PM   #4
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Re: Non Vented Vaulted Roof Assembly


No venting required for this kind of assembly.

Though most people in the know would recommend a vented assembly if at all possible and especially for a very cold climates, I think this situation really calls for the non-vented roof assembly.
You will find this assembly in the IRC 2012.

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Old 06-19-2015, 11:05 PM   #5
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Re: Non Vented Vaulted Roof Assembly


2x4 rafters means you will get r15 max with open cell insulation. I would go closed cell and get r21. Even that is not enough for NC climate.

180 sq ft... not much money for closed cell insulation.

What does the local building code official in NC have to say?

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Old 06-19-2015, 11:14 PM   #6
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Re: Non Vented Vaulted Roof Assembly


I'm on with Andy I think. Will they be running ac in the summer months? I would try to vent on top of the foam and then actually dense pack the rafter bays with cellulose. Really the only problem with the moisture is that the warmer moisture laden air cools down in the ceiling cavity and condenses and turns into water. If you aren't really seeing temperatures that cold then I'm not sure how much I'd worry about it. Closed cell foam would definitely work, but it can be costly. How air tight are they making this place
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Old 06-19-2015, 11:17 PM   #7
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Re: Non Vented Vaulted Roof Assembly


NC climate requires R30 for roof insulation. You will never achieve that in a 2x4 (3.5") depth.
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Old 06-20-2015, 05:38 AM   #8
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Re: Non Vented Vaulted Roof Assembly


Tiny houses don't fall under the IRC - that said biggest catch as stated is dew points & there is no way I would use OC (you will definitely need a dehumidifier unless they hardly stay in it). Andy's solution is the best cost wise, venting is not needed though unless you are looking at a ton of snow which NC doesn't get
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Old 06-20-2015, 05:46 AM   #9
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Re: Non Vented Vaulted Roof Assembly


Either way it goes, post pics please.
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Old 06-20-2015, 08:52 AM   #10
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Re: Non Vented Vaulted Roof Assembly


Another for CC on the deck.
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Old 06-20-2015, 10:24 AM   #11
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Re: Non Vented Vaulted Roof Assembly


Thanks for the replies. I think they're going to go with closed cell, as I've been informed that they will exceed height regulations by adding any materials to the roof decking.

That being said, does anyone still suggest a dehumidifier, or is it not needed with the closed cell?
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Old 06-20-2015, 12:14 PM   #12
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Re: Non Vented Vaulted Roof Assembly


Flash & Batt the thing. Spray closed cell foam on the the underside of the roof 1" thick and then install some fiberglass insulation. The job is so little, you could just spray the thing and not have the laborcosts associated with the fiberglass install...
Just tryin to help.
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Old 06-20-2015, 06:04 PM   #13
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Re: Non Vented Vaulted Roof Assembly


Spray the ceiling with closed cell. If they spray the walls with closed cell it will solidify the structure. Just some food for thought since it will be bouncing down the highway.
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Old 06-20-2015, 08:36 PM   #14
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Re: Non Vented Vaulted Roof Assembly


Your friends would be well advised to check with Building Inspector in county where they plan taking this thing. It might save them a whole bunch of heartbreak. My take is that unless parked at a campground, it will be considered stick-built with all inspections and codes that entails. People who have talked to me about tiny houses have charmed notion that they will only need a tiny septic system and a tiny well to go with it. Best of luck.
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Old 06-21-2015, 09:31 PM   #15
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Re: Non Vented Vaulted Roof Assembly


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Originally Posted by JesseCocozza View Post
That being said, does anyone still suggest a dehumidifier, or is it not needed with the closed cell?
I don't think any one of us could answer that. What is the humidity level in that area? Closed cell or not, humidity is always an issue.

Since it is 180sq ft... you would only need a small dehumidifier if you cannot control it. I would wait to purchase one until you see what you need. You may even need a humidifier.
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Old 06-21-2015, 10:27 PM   #16
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Re: Non Vented Vaulted Roof Assembly


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I don't think any one of us could answer that. What is the humidity level in that area? Closed cell or not, humidity is always an issue.

Since it is 180sq ft... you would only need a small dehumidifier if you cannot control it. I would wait to purchase one until you see what you need. You may even need a humidifier.

Yeah, I understand where you're coming from. I'm just trying to point them in the right direction because I have no experience in building envelopes in colder climates. After doing some research and learning about the accumulation of condensation in the open cell foam it's got them worried about their sheathing rotting out. As far as I know it's not an issue here because we don't experience the cold winters. Open cell is SOP here.
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Old 06-22-2015, 01:40 PM   #17
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Re: Non Vented Vaulted Roof Assembly


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Originally Posted by JesseCocozza View Post
That being said, does anyone still suggest a dehumidifier, or is it not needed with the closed cell?
Kind of figured on the height - as for a dehumidifier, as stated above it is almost a given unless they spend very little time in there (all seasons). Not only do you have exterior humidity to concern yourself with, you also have what the body normally gives off plus the usual showers, cooking, etc... In the case of closed cell it is probably needed even more as you won't have normal air leakage to help pull any out
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Old 06-22-2015, 07:36 PM   #18
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Re: Non Vented Vaulted Roof Assembly


Depending on how tight the rest of the place is they might have a ton of leakage in the walls and floor. Regardless in a building that size, a humidifier will dry the interior out pretty quick and might even pull humidity in through leaks. Are they trying to get this thing air tight? Are they planning on a blower door test? Is an ERV even made for something that small? Because I might consider an ERV for controlling humidity. There are a lot of variables that are inknown
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Old 06-22-2015, 09:45 PM   #19
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Re: Non Vented Vaulted Roof Assembly


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Depending on how tight the rest of the place is they might have a ton of leakage in the walls and floor. Regardless in a building that size, a humidifier will dry the interior out pretty quick and might even pull humidity in through leaks. Are they trying to get this thing air tight? Are they planning on a blower door test? Is an ERV even made for something that small? Because I might consider an ERV for controlling humidity. There are a lot of variables that are inknown

This was something I was pondering previously. The walls and floor are / will be insulated with packed shredded denim. So I'd image they will have some air leakage there.

I had no idea what a ERV was until I googled it. Seams like it might be a viable option if they can find space for it and if it's even needed.

I doubt they'll be preforming a blower door test.

Again, I thank you guys for your insight on all of this, as it's a departure from standard practices that I'm accustomed to here in SW Florida.
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Old 06-23-2015, 07:39 AM   #20
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Re: Non Vented Vaulted Roof Assembly


MW an HRV is actually the better option in humid climates for normal size houses (especially during the summer months as an ERV increases humidity loads as compared to an HRV, during winter an ERV is better at keeping some humidity in)

I had considered mentioning it but either would probably be overkill in such a small house (esp. as I don't think they make one that small) & just going in or out a few times would replace a lot of the air.

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